This invention relates to the redistribution of polycarbonates, and more particularly to a redistribution method employing a tetraorganophosphonium hydroxide as catalyst and an agent for decomposition of said catalyst.
Polycarbonates have traditionally been prepared by an interfacial method involving the reaction of a dihydroxyaromatic compound with phosgene, or a melt method in which the phosgene is replaced by a diaryl carbonate. In recent years, however, a redistribution process for use with polycarbonates has been developed. This process, described in U.S. Pat. Nos. 5,414,057 and 5,459,226, comprises heating an already formed polycarbonate in the presence of a redistribution catalyst to produce a polycarbonate with a different, generally lower, molecular weight. The method is highly useful commercially because it permits the preparation from monomeric materials of a single high molecular weight polycarbonate, which may then undergo redistribution to yield a spectrum of lower molecular weight materials which can be custom designed depending on their intended uses. A similar method, applicable to polyestercarbonates, is disclosed in U.S. Pat. No. 5,652,312.
A broad spectrum of redistribution catalysts is disclosed in U.S. Pat. No. 5,414,057, and the use of tetraorganophosphonium carboxylates as catalysts is disclosed in U.S. Pat. No. 5,459,226. Another excellent class of catalysts, disclosed in U.S. Pat. No. 5,567,802, is the tetraorganophosphonium hydroxides. They are frequently characterized by high efficiency. However, according to the procedure described in that patent the tetraorganophosphonium hydroxide remains in the polycarbonate after redistribution and can cause hydrolytic, melt and color instability.
There is a need, therefore, for materials which can be incorporated into redistribution mixtures containing tetraorganophosphonium hydroxides as catalysts and which cause decomposition of the catalyst after its function has been performed. Thus, such materials would have two requirements: to function only after redistribution has taken place and to cause efficient decomposition of the catalyst to harmless materials.